A “nanotaper”, or “inverse taper”, is generally defined as a terminating portion of a core of a high-contrast index waveguide that is used to facilitate efficient coupling between a single-mode optical fiber (for example) and an ultrathin, single-mode high-contrast index optical waveguide. For the purposes of the present invention, an “ultrathin” waveguide is defined as having a typical width on the order of approximately 0.5 μm and a typical thickness on the order of approximately 0.25 μm. In a typical device construction, the lateral dimension of the portion of the nanotaper proximate to the core of the high-contrast index waveguide approximately matches the width of the core. The lateral dimension of the nanotaper decreases monotonically along the direction of light propagation until it reaches a small value associated with a “tip” (i.e., that portion of the nanotaper distal from the core of the high-contrast index waveguide). The tip portion represents the point at which light first enters the high-contrast index waveguide for an “entry” nanotaper, or the point at which light first exits the high-contrast index waveguide for an “exit” nanotaper.
In some prior art nanotapers, the device is cleaved such that the tip position essentially coincides with a cleaved edge. Light is then launched directly into the tip of an entry nanotaper, or extracted directly from the tip of an exit nanotaper. Alternatively, in other prior art nanotapers, the position of the tip is located inside the device, away from the cleaved edge; an auxiliary waveguide is then used to transmit light from the cleaved edge to the tip of the nanotaper. The auxiliary waveguide generally comprises larger dimensions and a lower refractive index than the ultrathin waveguide, so that maximum coupling efficiency into the optical fiber is achieved since the mode size and numerical aperture of the auxiliary waveguide are much closer to the fiber parameters than the ultrathin waveguide parameters. The core of the auxiliary waveguide may comprise a polymer-based material with a refractive index on the order of 1.5–1.6. Other materials, such as silicon oyxnitride, doped silicon dioxide, etc. may be used to form the core region of the auxiliary waveguide.
In particular, a prior art nanotaper coupler usually comprises a pair of waveguide sections. A first, larger-dimensioned waveguide section is generally disposed with respect to a second, smaller-dimensioned waveguide section such that a portion of the first section overlaps a portion of the second section, forming a “mode conversion region”. One exemplary overlap geometry is disclosed in U.S. Patent Publication 2004/0057667, where FIGS. 1 and 2 illustrate this geometry in a top view and side view, respectively. As shown, reference numeral 1 denotes an ultrathin single-mode waveguide, reference numeral 2 denotes a mode field size conversion region, reference numeral 3 denotes an auxiliary waveguide section, and reference numeral 4 denotes a nanotaper. Within mode field size conversion region 2, nanotaper 4 has a lateral dimension that starts at a relatively small value at tip 5 (often 50–150 nm), and then tapers outward to the final desired waveguide dimensions associated with ultrathin waveguide section 1. The thickness of nanotaper 4 remains relatively constant along mode field size conversion region 2.
The mode size associated with tip 5 of the nanotaper 4 is “large” (due to the weak confinement of the light) and shrinks as nanotaper 4 expands in size, providing tighter confinement of the light as the effective refractive index increases along the length of the nanotaper. This effect facilitates the required mode conversion into the smaller mode associated with ultrathin single-mode waveguide 1. As shown, light is launched into an endface 6 of auxiliary waveguide section 3 (perhaps from an optical fiber—not shown), where it propagates along unimpeded until it encounters tip 5 of nanotaper 4 in mode conversion region 2. At this point, the light beam is transferred from the relatively low effective index layer 7 of auxiliary waveguide section 3 to the relatively high effective index ultrathin waveguide 1 with low loss, since the mode size is gradually reduced along the extent of the taper.
While these and other prior art nanotaper couplers have been successful in bringing light from an optical fiber into an ultrathin single-mode waveguide, there are limitations in how they may be employed. Perhaps the greatest hindrance in utilizing the prior art nanotaper couplers is the “end fire” coupling requirement; that is, an optical fiber (or other waveguide) must provide a signal that couples through a sidewall (endface) of an optical substrate and into the nanotaper. Such a coupling arrangement requires careful preparation of the sidewall surface in terms of cleaving and polishing (to reduce scattering losses), followed by the application of a anti-reflective (AR) coating. All of these preparation activities are expensive and time-consuming. A larger problem associated with the “end fire” coupling is the fact that only the edge of the substrate may be used for coupling; the remainder of the wafer surface is unavailable for use with a conventional nanotaper coupler.
Additionally, the larger auxiliary waveguide section often requires the use of material several microns thick (in order to establish a low-loss interface to the fiber), where these dimensions are not compatible with conventional CMOS processing techniques.
Thus, a need remains in the art for the development of a silicon nanotaper coupler that is more robust and can be used as a coupling device at virtually any location across a wafer surface.